296 W. LeConte Stevens—Notes on Physiological Optics. 
mer, however, 3", with that expressed in the curve A A’, 58™, 
for the same negative optic angle, and the same real distance, _ 
50™, it is seen that the change of conditions has produced a 
great change in the unconscious interpretation of the retinal 
image. In both cases the facts contradict Brewster's theory of 
triangulation. Brewster* himself noticed that when the com- 
bined image is small in comparison with the whole field of 
view, it did not appear at its calculated distance, even with 
convergence of axes, and to get rid of the disturbance due to 
comparison he resorted to large surfaces with geometrical pat- 
terns, but evidently without suspecting that optic divergence 
in viewing them was possible. 
hile the curve A A’, fig. 2, represents the result of exper- 
iment upon myself alone, similar results have been obtained 
from the examination of several other persons. In each case 
two curves approach more nearly to coincidence if the ob- 
server is well practiced and at the same time presbyopic, so 
that ciliary adjustment interferes less with the suggestion due 
to axial adjustment. 
ere are several considerations which interfere still further, 
er ne 
mates of each observer will be found to be affected with a 
nearly constant error, as has been shown by the experiments of 
Helmholtz and Wundt 
mal vision, furthermore, the dissociation between axial and 
focal adjustments is usually not instantaneous, and for strong 
